What Causes Prolonged Periods With Spotting And Brown Discharge?

I am on Microgynon 30 and have been for the past three months after changing from Dianette.
I have been fine the first two months however, i have been taking my pill everyday and at the same time, but about 6 days ago i began spotting and it has gradually got worse over the days and its a brownish colour. i carried on taking my pill and took the last one for this month today. Im just concerned as i have never had this before.